1. Field of the Invention
The present invention relates to a positive resist composition and to a pattern forming process using the same.
2. Description of the Related Art
It is well known that as LSI progresses toward a higher integration and a further acceleration in speed, a miniaturization of a pattern rule is required. Accompanying with this trend, an exposure method and a resist material are drastically changing. In particular, when a lithography of a pattern with 0.2 μm or less is carried out, KrF or ArF excimer laser beams, an electron beam, or the like are used as an exposure light source, and a chemically amplified type showing a good sensitivity to those high energy beams and giving a high resolution is used for a photoresist.
Resist compositions are classified into a positive type where exposed areas are made to be solvable and a negative type where exposed areas are left as a pattern, and that one of them is selected and used, which can be more conveniently handled depending on a required resist pattern. Further, chemically amplified positive resist compositions are each typically configured to contain: polymer, which is insoluble or poorly-soluble in an aqueous alkaline developer, and which is decomposed by an acid and turned thereby to be soluble in an alkaline developer; and an acid generator to be decomposed by exposure light to produce an acid; as well as a basic compound for restricting diffusion of the acid produced by the typical exposure.
Negative resist compositions of a type configured to adopt phenol units as alkali-soluble units constituting a polymer to be dissolved in the aqueous alkaline developer, have been numerously developed, and particularly for exposure by a KrF excimer laser light. Since phenol units of these compositions do not exhibit light transmittivity in the case of exposure light having wavelengths in a range of 150 to 220 nm, the compositions have not been used for ArF excimer laser light. However, such negative resist compositions have been recently noticed again as ones for EB or EUV exposure which are exposure methods for obtaining finer patterns, and reports thereof are found in Japanese Patent Laid-open (Kokai) Nos. 2006-201532, 2006-215180 and 2008-249762, and the like.
Further, examples of characteristics to be demanded for resist compositions in the case of the resist elaboration as noted above, include not only a higher resolution which is a fundamental performance of a resist, but also a higher etching resistance. This is because, progressively finer patterns necessitate to progressively decrease thicknesses of resist films. Known as one technique for obtaining such a higher etching resistance, is a method, which also has been disclosed in Japanese Patent Laid-open (Kokai) No. 2008-249762, to introduce, as subsidiary components of the polymer: a polycyclic compound such as indene and acenaphthylene, which includes an aromatic ring(s) and a non-aromatic ring(s), and in which the non-aromatic ring has a carbon-carbon double bond conjugated with the aromatic ring; into a polymer having hydroxystyrene units.
In turn, as polymers for positive resists, it has been proposed to use a polymer having only indene structures such as described in Japanese Patent Laid-open (Kokai) No. 2004-149756, while another method has been proposed in Japanese Patent Laid-open (Kokai) No. 2006-169302 to use units having acenaphthylene structures by combining them with a hydroxystyrene derivative.
However, in trying to conduct such a fine processing that gaps between line widths are decreased down to as narrow as 50 nm which is demanded in the presently most-advanced processing technique, bridges are caused between pattern lines even by variously conducting a fine control by adopting a polymer system which has been conventionally presented for a positive resist composition, thereby making it difficult to form a fine pattern.